Diazo sensitized papers have been used for a long time for making duplicate copies of originals, normally by contact printing and the subsequent development of the exposed diazo paper in an aqueous ammonia vapor atmosphere. In such an application, resolution requirements and development times are not critical. More recently diazo sensitized films have received increasing attention as an ideal medium for making microfilm or microfiche masters and, perhaps more importantly, duplicates thereof because of the relatively low cost of such film, its high resolution capability, etc. For such applications, however increasingly stringent demands are made on the film developing process, particularly as to the speed with which it can be accomplished so as to enable an efficient, high volume production of diazo film copies from a master, for example.
In this regard, problems have been encountered in the past. In order to attain short development times for diazo film, it was heretofore thought necessary that the development takes place in a high pressure ammonia atmosphere. In general, the pressures that were considered necessary are substantially above, e.g., several times the atmospheric pressure and they ranged up to as high as 1,000 psi or more. For example, U.S. Pat. No. 3,411,906 speaks of ammonia pressures in the range of between 50 and 1,000 psi. Little attention was paid to the actual design of the developing chamber into which the ammonia is introduced other than to maintain the volume relatively small for the obvious expedient of limiting the amount of ammonia that is expended in the developing process.
From a practical view, however, such high pressure requirements represent severe drawbacks, particularly in connection with a continuously operating diazo film developer since the exposed film must be transported from the exterior into the high pressure atmosphere. For one, ammonia leakage is quite unacceptable because of its noxious odor and the potential health hazard it represents if present in appreciable concentrations. Further, it is notoriously difficult to seal a pressurized chamber if continuous access to its interior is required unless the ammonia in the chamber is evacuated each time a film is inserted therein or withdrawn therefrom. This, however, is not compatible with a high speed, high volume operation.
Although the above-referenced U.S. patent does not concern itself with the actual chamber construction and is not concerned with the above summarized difficulties of operating it, another U.S. Pat. No. 3,364,833 proposes the construction of a diazo film developing device comprising a sealed chamber defined by a base having a cavity dimensioned to receive the film and a cover that is bolted and sealed against the base. The space of the cavity is kept as small as possible and once it is sealed, the air therein is evacuated and replaced with high pressure ammonia to develop the film. Although this device no doubt assures the full and complete development of the film and, if operated along the lines suggested in the earlier referenced U.S. patent yields short development times, the insertion and removal of the film from the cavity must be manually performed and surely exceeds the development time for the film by a very large factor. Thus, the device disclosed in U.S. Pat. No. 3,364,833 may be ideally suited for developing an individual diazo film from time-to-time; but it is unsuitable for continuous, large volume operations.
Thus, in spite of the advantages afforded by diazo film for high resolution, high volume applications such as for the duplication of microfiche masters, there is presently no technologically feasible device for economically mass developing such film.